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Re: [dm-devel] [PATCH 00/24] dm-raid456 support using md/raid5.c, now with dirty-log



On Wed, 16 Jun 2010 13:26:27 +0200
Heinz Mauelshagen <heinzm redhat com> wrote:

> On Wed, 2010-06-16 at 09:45 +1000, Neil Brown wrote:
> > Hi Heinz,
> > 
> > On Tue, 15 Jun 2010 15:23:26 +0200
> > Heinz Mauelshagen <heinzm redhat com> wrote:
> > 
> > > 
> > > Neil,
> > > 
> > > for missing devices we (re)load the table with error mapped device(s) in
> > > their place rather than identifying them with a special char/string.
> > > Did you go for the later in order to avoid some MD hassle with error
> > > targets being accessed by the MD personalities? If not so, we can drop
> > > the special '-' char support to identify missing devices.
> > 
> > When raid5 determines that a device has failed and so it will not write to it
> > again, it must be sure that the failure is record in the metadata before it
> > proceeds with that decision not to even try writing to the failed device.
> 
> Yes, that's the mandatory order to avoid data loss.
> 
> > Otherwise a crash/restart before the metadata was safely updated would result
> > in corruption.
> > 
> > This means that it must be possible for user-space to explicitly tell the
> > raid5 that a device is known to be faulty.
> > Doing that through the constructor seems the natural way to do it.
> 
> If /dev/mapper/error with an error target mapping would be used instead,
> would that cause consistency troubles to the MD personality if accessing
> those rather than the NULL device solution with the '-' argument you
> have now?
> If not so, we could drop the '-' support, which I'm aiming at.
> 

I would need to differentiate between /dev/mapper/error (where errors are
expected) and any other device (where errors are not expected).  How do you
propose I do that?

> > 
> > > 
> > > I'm thinking about adding a ctr wrapper to allow us to keep the "raid45"
> > > ctr interfaces semantics (ie. the arguments) and the new interface to
> > > "raid456" if you don't have objections. That would be implemented by 2
> > > targets being registered implemented in one module.
> > 
> > I have no strong objections, though having two targets that do almost the
> > same things seems rather ugly.
> 
> Keep in mind it'll only be another target_type structs, another
> raid_ctr() function plus another (de)registration of the respective
> target. The ctr functions for raid45 and raid456 can share moset of the
> code anyway.

Sure.

> 
> > 
> > Is there existing published code that uses the extra arguments to raid45?
> 
> Yes, dmraid with a list of RAID5 supporting metadata format handlers.

I just had a look at the latest dmraid from CVS and the only place that I can
find where a raid45 target is created is in lib/activate/activate.c in
_dm_raid45_bol.
This only uses arguments that my code understands.
What am I missing?

Thanks,
NeilBrown


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